The present invention relates to a method for melt-decontaminating a metal contaminated with a radioactive. substance, by adherence of a very small amount of the radioactive substance on the metal in nuclear facilities, which comprises melting the contaminated metal, adding a flux to the molten metal to combine the flux with the radioactive substance and separating the combined radioactive substance from the metal. In particular, it relates to a flux composition suitable for removing a radioactive substance as a contaminant from a zircaloy used as a cladding tube for a nuclear fuel rod.
A zircaloy is an alloy comprising zirconium (Zr) and tin (Sn) and other metals added thereto. By virtue of the minimum neutron capture cross section, zirconium in the form of an alloy with, as described above, a very small amount of another metal incorporated into zirconium in order to improve properties, such as mechanical strength, is used as a cladding tube for a nuclear fuel rod. A spent fuel rod is separated into a fuel portion and a cladding tube which are then processed respectively. In this case, a transuranium substance, such as plutonium (Pu), is adhered onto the cladding tube although the amount of the adhered transuranium substance is very small. That is, the cladding tube is in the state of being contaminated with a radioactive substance. Decontamination by removing the radioactive substance from the zircaloy enables the zircaloy to be reusable and, in addition, can offer various advantages in storage of the zircaloy.
One known method for decontaminating a metal contaminated with a radioactive substance comprises melting a contaminated metal, adding a flux or a slagging agent to the melt to combine the additive with the radioactive substance, and separating the radioactive substance from the contaminated metal (see, for example, Japanese Patent Laid-Open No. 61-26898/1986).
In the conventional melt decontamination, iron (Fe),for example, is decontaminated, and an Inorganic oxide, such as silicic acid (SiO.sub.2) or calcia (CaO), is generally used as the flux in this case.
Since zircaloy has a melting point as high as 1850.degree. C., difficulties are experienced in melting the zircaloy in a conventional melting furnace. In this case, the use of a floating-type melting apparatus, wherein a molten metal is floated so as not to be brought into direct contact with a crucible (see, for example, Japanese Patent Laid-Open No. 6-96852/1994), enables the decontamination to be carried out in the same manner as described above even in the case of a zircaloy having a high melting point.
Since, however, the melting point of the above-described flux is so low that the flux is evaporated at the melting point of the zircaloy, the flux cannot be chemically reacted with the radioactive substance efficiently. Further, since the zircaloy has a high chemical activity, even though the flux could be chemically reacted, it is unfavorably reacted with zirconium in the zircaloy, which inhibits the reaction of the flux with the radioactive substance, making it impossible to attain a contemplated decontamination effect.